High-pressure and high-temperature (HPHT) gas reservoirs are defined as reservoir having pressures greater than 10,000 psia and temperatures over 300 F. Modeling the performance of these unconventional reservoirs requires understanding of gas behavior at elevated pressure and temperature. An important fluid property is gas viscosity which is a key factor in modeling of gas mobility in the reservoir that can have a significant impact on reserves estimation during field development planning. Accurate measurements of gas viscosity at HPHT conditions are both extremely difficult and expensive. Thus, this fluid property is typically estimated from published correlations that are based on laboratory data. This book reviews the databases of hydrocarbon gas viscosity that are available in the public domain, and discusses the validity of published gas viscosity correlations based on their applicability range. Then the full process of measuring the gas viscosity at HPHT is well explained. Step by step developing an HPHT gas viscosity correlation and running a sensitivity analysis on the accuracy of the most well-known gas viscosity models at HPHT is discussed as well.